Passive fiber-optic component

ABSTRACT

A passive fiber optic component, in which two or more fibers are each bared at one end by removal of the outer coating of the fiber. The bare portions of the fibers are etched to produce cylindrical end portion which adjoins a conical portion. Subsequently, the fibers are arranged with their etched portions in a capillary tube which is sealed at one end. The tube is then evacuated and is fused with the etched portions of the fibers to form a solid rod with a rotationally symmetric distribution of the end portions of the fibers. The fibers are etched to such a diameter that after fusion of the fibers with the tube, the fused fiber ends have a circular cross-section substantially equal to the cross-section of a single fiber core. An end face is formed on the rod by cleaving or by grinding, and by polishing to obtain a fused fiber head. The fiber head forms a fiber optic component itself, or forms a basic element for a great number of fiber optic components such as splitters and couplers.

This is a division of application Ser. No. 853,309 filed Apr. 17, 1986,now U.S. Pat. No. 4,698,084 issued Oct. 6, 1987.

BACKGROUND OF THE INVENTION

The invention relates to a method of manufacturing a passive fiber opticcomponent. The component comprises at least two optical fibers, eachhaving a core of core glass, a cladding of cladding glass with arefractive index lower than that of the core glass, and an outercoating.

In the method, a part of each fiber is bared by removal of the outercoating over a given length from one end of the fiber. The bare parts ofthe fibers are subjected to an etching treatment whereby a portion ofthe etched part of each fiber is given a conical shape. The etchedportions of the fibers are then arranged against each other in a tube ofa glass having a refractive index lower than that of the core glass ofthe fibers. By applying heat, the tube is fused to the fibers and thebare parts of the fibers are fused together. Finally, the fused fiberbundle is provided with a polished end face.

A method for manufacturing a fiber optic component is described, forexample, in U.S. Pat. No. 4,291,940 and in European Patent Application0,123,396. These publications describe the manufacture of couplersaccording to a hot processing method, the so-called fused biconicaltaper technique. In this method, two fibers are twisted and are thenheated and stretched in such a manner that a coupler with a symmetricalbiconical configuration is obtained. In the biconical taper method, therisk of damage of the fiber and of deformation of the fiber core iscomparatively high. The method is only suitable for the manufacture ofcertain types of components, is not readily reproducible, and is notsuitable for use in mass production. Furthermore the input fiber can berecognized on the output side, which means that there is no uniformdistribution of the input power.

The first method discussed above is described in British PatentApplication 1,427,539 (corresponding to U.S. Pat. No. 3,933,455). Inthis method the fibers are not twisted. The fibers are drawn downtogether with the glass tube in which they are inserted to provide atapered zone from which interstices between the fibers have beeneliminated. The presence of a taper in this zone means that the diameterof the end face has to be selected by arranging it at an appropriatedistance along the length of the tapered zone. Considering the corediameter of the fibers to be treated (50 μm or smaller), it will bedifficult to select the correct diameter of the end face within closetolerances. The taper angle is dependent on several parameters and mayvary from fiber bundle to fiber bundle.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method of manufacturingpassive fiber optical components, which method is flexible and by meansof which different types of components can be manufactured on a largescale in an economical manner and with the required precision.

According to the invention, a fiber optic component is manufactured byetching the bare part of each fiber to produce a cylindrical end portionwhich adjoins the narrow end of the conical portion. The tube is acapillary tube which is sealed at one end. The tube and at least thecylindrical end portions of the fibers (inserted in the tube) aresoftened by heat while the tube is evacuated so that the wall of thetube collapses against the fibers. While maintaining its circularcross-section (under the influence of surface tension), the capillarytube deforms the end portions of the fibers to eliminate the intersticesbetween them and to give these portions of the fibers collectively acircular cross-section. Together with the tube, the fibers form a solidrod of circular cross-section. The tube is fused by the heat to the endportions of the fibers.

Subsequently, the sealed end of the rod is removed to form on the rod anend face on which the ends of the end portions of the fibers areexposed. The diameter of the circular cross-section of the end portionsafter the deformation is substantially equal to the diameter of the coreof a single fiber.

Finally, the end face is polished and finished to obtain a fused fiberhead.

By means of the method according to the invention, different opticalcomponents (such as splitters, directional couplers, transmissive andreflective star couplers, multiple connectors and the like) can bemanufactured with the required micron accuracy in mass production and ina comparatively inexpensive manner.

The fiber head itself may constitute a fiber optic component or it maybe coupled with a single fiber or with another identical or similarfiber head to form a passive fiber optic component. Since the processparameters are known and controllable, and the product is accessible forinspection and verification, the process is also reproducible andsuitable for automatization. As the end portion of each fiber iscylindrical and as the final rod is also cylindrical (at least over acertain length) and has a predetermined diameter, the end face need nothave any particular diameter. The end face may be cut along thecylindrical part of the rod within large tolerances.

Due to the conical portion situated between the cylindrical end portionand the unetched fibers, a strong and abrupt bending of the fiber coresduring the fusion step is prevented. The cross-section of the fusedcylindrical fiber ends progressively increases toward the unetched fiberportions.

As the capillary tube with the fibers inserted therein is not drawn, thewall thickness of the capillary tube is not reduced. Therefore, at theend face the fused ends are surrounded by a relatively large annulararea formed by the wall of the capillary tube. This forms a largebonding surface.

During heating, the evacuated capillary tube will flow and shrink underthe influence of atmospheric pressure while maintaining its circularcross-section due to surface tension. The end portions of the fibers aredeformed into a rotationally symmetrical pattern within the shrunk tube.

The controlled reproducibility obtained in this way is essential formass, automated production. No external forces, which could damage thefibers, are exerted on the fibers.

In the method according to the invention, step index fibers,graded-index fibers, monomode fibers, or multimode fibers can be used.

Since the capillary tube serves as a cladding in the finished product,the tube should be made of a glass having a refractive index lower thanthat of the core glass of the fibers. Preferably, the refractive indexof the tube is equal to that of the cladding glass.

The glass of the capillary tube preferably has a softening temperatureslightly higher than that of the core glass. Given a suitable softeningtemperature and a suitable refractive index of the core glass of thefibers to be fused together, preferably the capillary tube is quartzglass.

The outer coating of the fibers generally consists of a syntheticmaterial, such as for example a UV curing acrylate.

As the fused fiber ends have a circular cross-section substantiallyequal to the cross-section of a single fiber core, a splitter is simplyassembled by gluing the single fiber to be coupled in a capillary tubehaving a diameter substantially equal to the diameter of the fiber head.In a known arrangement, the fiber head and the individual fiber arealigned with respect to each other so as to obtain the best compromisebetween a maximum total signal transmission and a uniform distributionof power over the output ports. Subsequently, at the coupling area, glue(for example UV curing glue of the correct refractive index) is added.After the distribution over the fibers has been verified and, as thecase may be, has been corrected, the UV curing glue is cured.Ultimately, the assembly is encapsulated in a suitable quartz tube, forinstance by epoxy.

For assembling a coupler it is sufficient to couple the end faces of twofused fiber heads obtained by the method according to the invention (andhaving the required number of fibers) to each other, as the diameters ofthe fiber heads and of the fused fiber ends are the same. If thecladdings in the cylindrical end portions of the fibers are completelyetched away, a loss-free coupler is formed with rotational symmetry andwith complete mixing, when using step-index fibers. When usinggraded-index fibers, a low-loss coupler is obtained with rotationalsymmetry and with only a limited amount of coupling, but the input fibercan be recognized.

If the claddings of the fibers are not entirely etched away, no orsubstantially no mixing occurs in the fiber head. If no mixing occurs atall, the degree of coupling can be completely controlled by relativerotation of the two fiber heads. With step index fibers, a symmetricaldirectional coupler is thus obtained with an arbitrary fixed or variablecoupling ratio and with low losses due to a remaining central line ofcladding glass.

If the claddings of the fibers are entirely etched away, there is a baselevel of coupling which depends upon the length of the fiber head. Avariable amount of coupling can be added by relative rotation of the twofiber heads. For graded-index fibers, the power is rather confined tothe core. Even if the claddings of the fibers are entirely removed,there is the possibility of incomplete mixing. However, by rotationallysymmetric positioning, a loss-free directional coupler with a lowcoupling ratio can be obtained. By rotation, the losses increase withthe coupling ratio due to the fact that the center lines of the fibersdo not coincide.

The same coupling ratio can be reached only by a larger angle ofrotation if the claddings are not entirely removed. The losses increasedue to the fact that the center lines do not coincide. With a couplingratio of 1:1, the loss is equal to the square of the loss of the two-waysplitter.

Couplers consisting of two, three or four input and output ports andmanufactured by the method according to the invention form a new classof products. Since such couplers with step-index fibers are loss-freeand can be used more widely than those with graded-index fibers, theadvantages obtained by the use of step-index fibers instead ofgraded-index fibers in networks, especially local area networks in whichmany passive fiber-optic components are required, are evident. Joiningan M-fiber head to an N-fiber head gives rise to components with specialproperties, which may be useful in fiber optic networks.

Star couplers form an extension of the aforementioned series ofcomponents and may be composed of graded-index fibers or of step-indexfibers. Since in a star coupler With step-index fibers a complete mixingtakes place, the input power is distributed equally over all the fibercores so that the individual fibers cannot be distinguished. In one suchstar coupler, for instance, two multiple fused fiber heads are coupledto each other directly by their end faces.

In a star coupler composed of graded-index fibers, the input power isnot distributed equally over all the fibers. Therefore, the two fiberheads have to be coupled to each other by means of a mixing element likea mixing rod or graded-index rod.

A fiber head obtained by means of the method according to the inventionprovides an ideal possibility for transmitting laser power. By using abundle of step-index fibers having at cladding entirely etched away fromthe free ends which are finished to form individual fiber heads, thepower of a single laser can be used for welding or soldering severalspots at a time. In the fiber head, the input laser power is mixed bycoupling in such a manner that all fibers transmit the same power at theoutput end. Furthermore the shape of the output head can be adapted tothe shape of the workpiece.

In the above embodiments of the method according to the invention, it isassumed that the fibers to be fused together fit into the capillary tubewith their bare non-etched cladding with a narrow tolerance and in aregular pattern. This assumption is limited to a maximum number of fivefibers.

When six or more fibers or a larger number of fibers are to beprocessed, it is not possible to pack the fibers tightly in thecapillary tube since space larger than a fiber remains at the center ofthe tube. In order nevertheless to be able to carry out the process, ina preferred embodiment of the method according to the invention thefibers are regularly distributed around the inner circumference of thecapillary tube and are supported there by a cylindrical supportingmember. The supporting member is arranged centrally in the capillarytube.

The fibers are adhered, via their bare claddings, to the inner surfaceof the capillary tube by a thermal pretreatment. Then the supportingmember is removed, and the tube and the etched end portions of thefibers are fused. During fusion of the capillary tube with the endportions of the fibers, the central space becomes filled by the fibers,which are each deformed into a sectorial cross-section. The deformedfibers are uniformly arranged in a symmetrical pattern around the innercircumference of the capillary tube.

It is an essential characteristic of the method according to the presentinvention that the products have a circularly symmetric distribution offibers. This implies that each segment radiates into the opposite endwith the same property so that there is no difference in the modeeffects whatever input fiber is excited and whatever output fiber ismeasured. Not only the energy is distributed equally over the fibers,but also the mode spectrum is the same for all fibers. This hasadvantages for following fiber optic components downstream in a network.

A fiber head manufactured by the method according to the invention caneven act as a fiber optic component, or as input and/or output end of abundle of fibers. In the embodiments already described, the fiber headconstitutes a standard fiber optic component which serves as a basicelement for a complete series of different composite fiber opticcomponents.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1a is an axial sectional view of an optical fiber, drawn to agreatly enlarged scale.

FIG. 1b is a similar view of the optical fiber after the etchingtreatment according to the invention.

FIGS. 2a, 2b and 2c are axial sectional views, drawn to an enlargedscale, of a capillary tube used in the invention, in successiveprocessing stages.

FIGS. 3a to 3i show successive steps in the manufacture of a splitter bymeans of the method according to the invention.

FIGS. 4a, 4b and 4c show successive steps of another embodimentaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For manufacturing a fiber optic component by means of the methodaccording to the invention, first a number of optical fibers as shown inFIG. 1a are cut to a desired length. In practice, the fibers may be cutto approximately 1 meter.

The optical fiber 1 shown in FIG. 1a comprises a glass core 3 having adiameter d1, a glass cladding 5 having a diameter d2, and an outerprotective coating 7 having a diameter D. The cladding 5 is made of aglass having a refractive index lower than that of the core glass. Thecoating 7 generally consists of a synthetic material, such as a UVcuring acrylate. A commonly used optical fiber has a diameter D of 250μm, while the cladding 5 has a diameter d2 of 125 μm, and the core 3 hasa diameter d1 of 50 μm.

For preparing the fibers, first the coating 7 is removed over a lengthof a few centimeters from one end of each fiber by immersing thisportion of the fiber in dichloromethane or by burning with a flame.Subsequently, the portions of the fibers thus bared are etched in such amanner that over a length of approximately 1 centimeter a cylindricaletched end portion 9 is obtained adjoining a conical etched intermediateportion 11. (FIG. 1b.) Conical portion 11 in turn adjoins the non-etchedbare cladding 5 having its original diameter d2.

Depending upon the desired properties of the fiber optic component to bemanufactured, the cladding 5 may be only partly removed at thecylindrical etched end portion 9 or may be entirely removed at thisportion (that is to say, removed to the interface between cladding andcore, in which case the core 3 retains its original diameter d1). Forsome applications, for example for the manufacture of a splitter havingat most four fibers, or with the use of graded index fibers, the etchingtreatment is continued until a part of the core is also etched away sothat the cylindrical etched end portion 9 has a diameter d3 which issmaller than the original diameter d1 of the core 3. This situation isshown in FIG. 1b.

The fibers are etched by immersing the fibers in a HF solution with aconcentration of, for instance, 50%. The conical etched portions 11 areobtained by moving the fibers up and down during etching; the stroke ofthis movement determines the length of the conical portion 11, which ison the order of millimeters to centimeters. The diameter d3 of thecylindrical etched end portion 9 is determined so that when the endportions of the fibers are subsequently fused together in the requirednumber they will together have a cross-section substantially identicalin shape and size to that of a single fiber core 3.

FIGS. 2a, 2b and 2c show successive steps in the processing of acapillary tube 21 required for carrying out the present example of themethod according to the invention. The capillary tube 21 is obtained bydrawing a long tube from a preform of quartz glass, then subdividingthis tube into capillary tubes 21 each having a length of about 3 to 6centimeters, approximately equal to the length of the bare portions ofthe fibers. The glass of the tube 21 has a refractive index lower thanthat of the core glass of the fibers.

The diameter d4 of the capillary duct 23 in each tube 21 is chosen sothat the bare portions of the required number of fibers will fit into itwith a small amount of clearance of about 10 μm. In view of the requiredaccuracy of shape and in order to obtain a sufficient mechanicalstrength of the tube 21 and of the ultimate product, a comparativelylarge wall thickness of 1 to 2 millimeters is chosen. For the usualproducts, the duct 23 has a diameter d4 of 260 to 400 μm, while the tube21 has a diameter d5 of 2.5 to 6 millimeters.

The tube 21 shown in FIG. 2a is formed with a funnel 25 at one end. Thefunnel 25 facilitates the insertion of the fiber ends into the capillarytube and offers a bonding surface for adhering the outer coating of thefibers to the capillary tube.

For receiving fibers whose cores are etched down to a smaller diameterthan the original diameter, and in order to obtain an optimumpositioning of the fiber ends the capillary duct 23 is provided at theend remote from the funnel 25 with a restriction 27, as shown in FIG.2b. The restriction can be obtained by heating and shrinking under theinfluence of surface tension or by drawing down. The restriction 27 isgiven a diameter d6 such that the cylindrical etched end portions 9 ofthe fibers will fit into it.

Subsequently, the tube 21 is sealed at the end with the restriction 27.FIG. 2c shows the finished capillary tube 21. However, the process ofrestricting and sealing the capillary tube 21 may also take place at alater stage.

Hereafter, an embodiment of the method according to the invention formanufacturing a splitter having four output ports will be described.FIGS. 3a and 3i schematically show the successive steps.

FIG. 3a shows the etching of four fibers 1 in an etching bath 31. Forthis purpose, the fibers 1 are fixed in a holder 33, which can be movedup and down.

FIG. 3b shows a fiber 1 after the etching treatment. Fiber 1 has acylindrical etched end portion 9 as already described. Fiber 1 also hasa conical etched portion 11, a bare non-etched clad portion 5, and afully coated portion 7.

The bare portions of the four fibers prepared in this manner are theninserted into the capillary tube 21, into which the fibers fit with anarrow tolerance. This is shown in FIG. 3c, which shows only a part ofthe capillary tube which is sealed already. As shown in FIG. 3d thefibers 1 are arranged with their bare parts in the capillary tube 21,the coating 7 terminating at the funnel 25.

Subsequently, the capillary tube 21 is connected to a vacuum chamber 37and is evacuated to a pressure lower than 10⁻¹ mbar, which stage isshown in FIG. 3e. The capillary tube 21 is then degassed by arranging itabove the furnace 39 shown in FIG. 3f. Furnace 39 consists of a graphitesleeve 41 which is flushed with N₂ and is heated by a high-frequencycoil 43.

Subsequently, the capillary tube 21 is introduced into the furnace 39,which is already at the fusion temperature of 1600° to 1800° C.Depending upon the temperature in the furnace, the time required forfusion is 1 to 10 minutes. At the correct temperature and heating time,the capillary tube 21 shrinks under the influence of the vacuum and theatmospheric pressure, while maintaining the circular cross-section dueto surface tension. The end portions 9 of the fibers are deformed into asymmetrical cross-sectional pattern of the kind shown in FIGS. 3g and3h. The capillary tube 21 is fused with the end portions 9 of the fibersto form therewith a solid rod 45.

The tube 21 is removed from the furnace and the outer coatings of thefibers are glued to the funnel 25. Subsequently, the sealed end of therod 45 is removed, preferably by scribing and cleaving. The ruptured endsurface of the resulting fiber head 47 is finished by grinding andpolishing to form the end face 46. FIG. 3h shows the finished fiber head47 with the end face 46.

As already described above, the fused end portions 9 of the fibers havea diameter and a cross-section substantially identical in shape and sizeto the diameter and the cross-section of a single fiber core.Consequently, as shown in FIG. 3i, for assembling a splitter 52, thefiber head 47 can be directly coupled and fixed to a fiber head 48comprising a single fiber 1 by a glue connection 49 in the manneralready described. The free end of the single fiber is inserted andfixed in a capillary tube 51 having the same external diameter as thefiber head 47. Subsequently the fiber heads 47 and 48 are inserted intoa quartz envelope 53 and are fixed therein by glue beads 54.

FIGS. 4a, 4b and 4c show the fusion of six or more fibers. It is notpossible to arrange six or more fibers in a dense packing withoutinterstices in the capillary tube 21. FIG. 4a shows that with, forexample, eight fibers, the packing is loose resulting in suchinterstices that fusion with a symmetrical cross-section pattern is notobtained.

As shown in FIG. 4b, this problem is obviated by arranging a temporarysupporting member 55, for example a tungsten wire, at the center of thecapillary tube 21, and by distributing the fibers in contact with oneanother around the inner circumference of the tube. By a thermalpretreatment, the fibers are adhered to the inner surface of the tube21. The supporting member 55 is then removed after which the fibers 9and tube 21 are fused. A fiber head 57 is produced in which the fusedend portions 9 of the fibers each have a sectorial cross-section. (FIG.4c.)

For manufacturing fiber heads having a reduced minimum length, thecapillary tube preferably is heated by means of a CO₂ laser. To this endthe sealed capillary tube is rotated, and a focused laser beam isdirected radially onto the capillary tube and is displaced along thetube in the axial direction thereof. Due to the concentrated localheating of the tube, the distance between the fused fiber ends and theouter coatings of the fibers (i.e. the length of the bare fiberportions) can be reduced considerably. A similar improvement can beobtained by using a small burner or torch, the small spot-like flame ofwhich also enables a concentrated local heating of the capillary tube.

In the embodiment described above, the sealed end rod 45 is removed byscribing and cleaving. The ruptured end surface of the resulting fiberhead 47 is finished by grinding and polishing. When after cleaving andafter grinding of the ruptured surface the fused fiber ends arefirepolished with a small H₂ O burner, an end face is obtained with onthe one hand a very smooth polished surface at the light transmittingarea, and on the other hand a relatively rough surface with good bondingproperties at the bonding area.

As already set out above the present method is suited for processinggraded-index fibers and step-index fibers as well. The embodimentdescribed deals with the processing of multimode fibers. However, thepresent method is not limited to the processing of multimode fibers.Experience has shown that due to the high degree of reproducibility andprecision obtained, the method is also suited for processing single modefibers.

What is claimed is:
 1. A passive fiber optic component comprising:aglass core having a refractive index and having a substantially circularcross-section; a glass cladding surrounding the core, said claddinghaving a refractive index less than the refractive index of the core andhaving a substantially circular cross-section; and at least two opticalfibers optically coupled to the core, each of said fibers having endportions with reduced cross-sections; wherein the glass core comprisesend portions of the optical fibers fused together to form a solid glasscore with no spaces between the fiber end portions, the fused endportions collectively having circular cross-section the diameter ofwhich is substantially equal to the diameter of the core of one of saidat least two optical fibers.
 2. A passive fiber optic componentcomprising:a glass core having a refractive index and having asubstantially circular cross-section; a glass cladding surrounding thecore, said cladding having a refractive index less than the refractiveindex of the core and having a substantially circular cross-section; andat least two optical fibers optically coupled to the core, each of saidfibers having end portions with reduced cross-sections; wherein: theglass cladding has a substantially cylindrical end portion with asubstantially constant diameter; and the end portion of the opticalfibers are fused together to form a solid glass core with no spacesbetween the fiber end portions, and the glass core has a substantiallycylindrical end portion with a substantially constant diameter.
 3. Apassive fiber optic component as claimed in claim 2, wherein the opticalfibers are multimode fibers.
 4. A passive fiber optic component inclaimed in claim 2, wherein the optical fibers are single mode fibers.5. A passive fiber optic component comprising:a glass core having arefractive index and having a substantially circular cross-section; aglass cladding surrounding the core, said cladding having a refractiveindex less than the refractive index of the core and having asubstantially circular cross-section; and at least two optical fibersoptically coupled to the core, said fibers having end portions withreduced cross-section; wherein: the glass core comprises the endportions of the optical fibers, said end portions being fused togetherto form a solid glass core with no spaces between the fiber endportions, the diameter of the circular cross-section of the fused endportions being substantially equal to the diameter of the core of one ofsaid at least two optical fibers, said glass core having a substantiallycylindrical end portion with a substantially constant diameter; and theglass cladding has a substantially cylindrical end portion with asubstantially constant diameter.
 6. An optical splitter comprising:afiber head having a glass core and a glass cladding surrounding thecore, said core and cladding having an end face, said core having adiameter at the end face; and an optical fiber having a core and acladding surrounding the core, said core and cladding having an endface, said core having a diameter at the end face substantially equal tothe diameter of the core of the fiber head, said end faces adjoiningeach other; wherein: the glass core of the fiber head has a refractiveindex, has a substantially circular cross-section, and has asubstantially cylindrical end portion with a substantially constantdiameter; the glass cladding of the fiber head has a refractive indexless than the refractive index of the core, has a substantially circularcross-section, and has a substantially cylindrical end portion with asubstantially constant diameter; the fiber head further comprises atleast two optical fibers optically coupled to the core of the fiberhead, said fibers having end portions with reduced cross-sections; andthe glass core of the fiber head comprises the end portions of theoptical fibers, said end portions being fused together to form a solidglass core with no spaces between the fiber end portions.
 7. An opticalcoupler comprising:a first fiber head having a glass core and a glasscladding surrounding the core, said core and cladding having an endface, said core having a diameter at the end face; and a second fiberhead having a core and a cladding surrounding the core, said core andcladding having an end face, said core having a diameter at the end faceequal to the diameter of the core of the first fiber head, said endfaces adjoining each other; wherein: the glass core of each fiber headhas a refractive index, has a substantially circular cross-section, andhas a substantially cylindrical end portion with a substantiallyconstant diameter; the glass cladding of each fiber head has arefractive index less than the refractive index of the core, has asubstantially circular cross-section, and has a substantiallycylindrical end portion with a substantially constant diameter; eachfiber head further comprises at least two optical fibers opticallycoupled to the core, said fibers having end portions with reducedcross-sections; and the glass core of each fiber head comprises the endportions of the optical fibers, said end portions being fused togetherto form a solid glass core having spaces neither between the fiber endportions.
 8. A passive fiber optic component comprising:a glass corehaving a refractive index and having a substantially circularcross-section; a glass cladding surrounding the core, said claddinghaving a refractive index less than the refractive index of the core andhaving a substantially circular cross-section; and at least two opticalfibers optically coupled to the core, said fibers having end portionswith reduced cross-sections, each end portion having a cross-sectionshaped substantially as a sector of a circle; wherein: the glass core isderived by a method wherein the outer coating of at least two fiberseach comprising a glass core having a refractive index, a glass claddingsurrounding the core having a refractive index less than the refractiveindex of the core, and an outer coating surrounding the cladding isremoved over a selected length from the end of the fiber to produce abare fiber end portion; the bare end portion of each fiber is etched toproduce a conical etched portion and an adjoining cylindrical etchedportion, the cylindrical etched portion having a substantially constantdiameter and adjoining the end of the fiber; the etched fiber endportions are arranged in the bore of a glass tube having a bore with asubstantially circular cross-section, which is closed at one end andwhich has a refractive index less than the refractive index of the coresof the fibers and the tube is evacuated; the evacuated tube and theetched fiber end portions therein are heated to collapse the tube aroundthe etched end portions without drawing the tube and to fuse the etchedfiber end portions to each other and to the tube whereby the heatedetched fiber end portions are deformed to eliminate spaces between theetched fiber end portions to produce a solid rod with a substantiallycircular cross-section and with a core of substantially circularcross-section; the closed end of the collapsed tube is removed to exposethe ends of the deformed fiber end portions; and the exposed ends arepolished to produce a fused fiber head.
 9. A passive fiber opticcomponent as claimed in claim 8, wherein:the glass cladding has asubstantially cylindrical end portion with a substantially constantdiameter; the end portions of the optical fibers are fused together toform a solid glass core with no spaces between the fiber end portions,and the glass core has a substantially cylindrical end portion with asubstantially constant diameter.
 10. A passive fiber optic component asclaimed in claim 9, wherein the optical fibers are multimode fibers. 11.A passive fiber optic component as claimed in claim 9, wherein theoptical fibers are single mode fibers.
 12. A passive fiber opticalcomponent as claimed in claim 8, wherein the etched fiber end portionsare regularly distributed around the circumference of the bore by amethod wherein:a cylindrical supporting member is provided in the bore,said etched fiber end portions being arranged around the cylindricalsupporting member; the tube and the etched fiber end portions are heatedto adhere the etched fiber end portions to the tube; and the cylindricalsupporting member is then removed.